A known method for the withdrawal of water uses a vertical double tube, uniformly provided with openings, via which water is drawn off by means of a subaqueous pump (or other withdrawal device). However, this method only provides a rather low withdrawal capacity if it is required that the water withdrawn shall be grit-free, i.e. if sand particles above a critical grain size are not also to be drawn off from the surrounding water-bearing stratum.
This method with the device, the so-called suction current collection (SCC), is known from German Offenlegungsschrift 2,401,327. According to the latter, the so-called collection element of the SCC consists of two coaxially disposed tubes of different diameter with uniform transverse slots over the variable length, the hollow cylindrical gap between the two tubes being filled with a fine-grain granulate. This construction is intended to achieve the result that the horizontal approach velocity at the critical point R.sub.K (FIG. 1) is approximately constant over the entire effective vertical length of the collection element, the so-called drainage length L.sub.E. As measurements have shown, a uniform horizontal approach velocity over the entire drainage length cannot be achieved with the collection element known from German Offenlegungsschrift No. 2,401,327. For a uniform approach velocity at the critical point R.sub.K (FIG. 2) over the drainage length of the collection element this has the result that for a certain delivery rate a certain quantity of fine-grain sand particles is still entrained and consequently drawn off.
It is therefore the object of the invention to provide, for a certain delivery rate, a method for the grit-free withdrawal of water from a well and also an associated device, and in doing this at the same time to reduce further the energy consumption compared to conventional water delivery without SCC as a result of a still lower groundwater depression in the well area.